Ethylene dimerization typically offers higher quality butene feeds for both propylene production via ethylene/butene disproportionation (metathesis), and alkylate production with isoparaffins, usually due to increased throughputs from the substantially butane-free feed.
A butenes product from ethylene dimerization can be fed to a propylene unit and an alkylation unit operated in parallel, but such systems typically suffer from one or more disadvantages. When a disproportionation unit is operated separately, the concentration of 2-butene in the recycled butenes usually is lower than the concentration of 2-butene in the butenes obtained from ethylene dimerization. The recycled butenes are believed to have an increased 1-butene concentration due at least in part to thermodynamic equilibrium. As a result, the propylene selectivity and/or the subsequent yield usually are lower than the propylene selectivity and/or the subsequent yield achieved with a fresh butenes feed from ethylene dimerization.
Moreover, when an alkylation unit uses butenes for an ethylene dimerization process, a de-ethanizer typically is required to remove unconsumed ethylene in order to reduce or avoid excessive sulfuric acid catalyst consumption during alkylation.
There remains a need for methods that overcome one or more of the foregoing disadvantages.